Acrylic acid is industrially important as a starting material for various synthetic resins, paints, plasticizing agents and the like. In recent years, its importance is still increasing, as a starting material for water absorbent resins. Acrylic acid is most generally produced by two-stage oxidation method, in which propylene is catalytically oxidized at vapor phase to yield mainly acrolein, and the acrolein is successively catalytically oxidized at vapor phase to yield acrylic acid. New production methods are also proposed recently, such as acrylic acid production by catalytic vapor-phase oxidation of acrolein which is obtained by dehydration of glycerin. Acrylic acid is now produced in all the world on a scale of several millions of tons per year, and the demand therefor as a starting material of water absorbent resins continues to expand. To cope with such increasing demand, a simple and general solution is to raise the productivity of acrylic acid by increasing the load of the starting material at the steady state of the catalytic vapor-phase oxidation.
The catalytic vapor-phase oxidation of acrolein, however, is exothermic, and the calorific value also increases when the acrolein load as the starting material is increased. Besides, during the period of start-up from the non-reacting condition up to immediately after attaining the prescribed reaction conditions, the catalytic activity is unstable, and when the acrolein load is rapidly increased at the start-up, abnormal heat generation in the catalyst layer(s) is apt to occur to give rise to local heat-generating sites (hot spots), inviting in consequence reduction in the acrylic acid yield due to the high temperature reaction and deterioration of the catalyst which is exposed to the high temperature. Such problems become even more serious when the vapor-phase oxidation is carried out under high load condition.
Thus, in the method of producing acrylic acid by catalytic vapor-phase oxidation of acrolein with molecular oxygen, a process enabling the production with higher stability and higher productivity or yield is in demand, and a number of proposals have been made also about contrivances for the start-up.
For example, the following patent documents 1 and 2 disclose a method in which the supply amount per unit time of the starting material is kept low for a fixed period at the start-up stage of the reaction.